Wash the cucumber and cut it into slices ¼-inch thick.
In a large bowl, whisk the sour cream, mayonnaise, dill, vinegar, sugar and ⅛ teaspoon salt.
Add the cucumbers and onions if using and gently toss well to coat.
Refrigerate for 1 hour before serving.
Taste and season with additional salt and pepper and sprinkle with fresh additional dill if desired.
